To determine which DNA will melt at the lowest temperature, we need to analyze the number of hydrogen bonds present in each DNA segment. The melting temperature of DNA is influenced by the number of hydrogen bonds between the complementary base pairs: adenine (A) pairs with thymine (T) via 2 hydrogen bonds, and guanine (G) pairs with cytosine (C) via 3 hydrogen bonds. The more hydrogen bonds present, the higher the melting temperature.
### Step-by-Step Solution:
1. **Identify the Base Pairs**:
- For each DNA segment, identify the pairs of bases (A-T and G-C).
2. **Count the Hydrogen Bonds**:
- For each A-T pair, there are 2 hydrogen bonds.
- For each G-C pair, there are 3 hydrogen bonds.
3. **Calculate Total Hydrogen Bonds for Each Option**:
- **Option 1**:
- Base pairs: A-T, A-T, T-A, A-T, A-T, A-T (6 A-T pairs) → 6 x 2 = 12 hydrogen bonds
- Base pairs: G-C, G-C, G-C (3 G-C pairs) → 3 x 3 = 9 hydrogen bonds
- Total = 12 + 9 = **21 hydrogen bonds**
- **Option 2**:
- Base pairs: A-T, A-T, A-T, A-T (4 A-T pairs) → 4 x 2 = 8 hydrogen bonds
- Base pairs: G-C, C-G, G-C, C-G (4 G-C pairs) → 4 x 3 = 12 hydrogen bonds
- Total = 8 + 12 = **20 hydrogen bonds**
- **Option 3**:
- Base pairs: A-T, T-A, T-A, A-T, T-A (5 A-T pairs) → 5 x 2 = 10 hydrogen bonds
- Base pairs: G-C, C-G, G-C (3 G-C pairs) → 3 x 3 = 9 hydrogen bonds
- Total = 10 + 9 = **19 hydrogen bonds**
- **Option 4**:
- Base pairs: A-T, A-T, A-T, T-A (4 A-T pairs) → 4 x 2 = 8 hydrogen bonds
- Base pairs: G-C, C-G, G-C, C-G (4 G-C pairs) → 4 x 3 = 12 hydrogen bonds
- Total = 8 + 12 = **20 hydrogen bonds**
4. **Compare Total Hydrogen Bonds**:
- Option 1: 21 hydrogen bonds
- Option 2: 20 hydrogen bonds
- Option 3: 19 hydrogen bonds
- Option 4: 20 hydrogen bonds
5. **Determine the Lowest Melting Temperature**:
- The DNA with the lowest number of hydrogen bonds will melt at the lowest temperature.
- **Option 3 has the least number of hydrogen bonds (19)**.
### Conclusion:
The DNA that will melt at the lowest temperature is from **Option 3**.
